Page:Popular Science Monthly Volume 29.djvu/83

Rh the history of the universe, a work which astronomy teaches us to read. On one of these pages, that has already been in part deciphered, is recorded the destiny of our planet.

It is, then, not surprising that astronomers seek to gain possession of as many reliable copies of such leaves from this history as possible; in other words, seek to own as exact and extensive star-maps as will include the very smallest luminous points in the heavens. What untold work the compiling of such charts entails may well be imagined; indeed, this is a task which is almost beyond human power. The chart from which the above picture is a copy was compiled at the observatory at Paris, and work at the same has already been continued for many decades. For years past, the two brothers, Paul and Prosper Henry, have been engaged in this exacting undertaking; but, notwithstanding the great experience which they in the course of time had gathered, their task almost came to a sudden end in the year 1884. At that time, while pursuing their observations, they came to that region of the heavens traversed by the milky-way. As is well known, the mild, lambent light of the milky-way is caused by a conglomeration of countless millions of stars placed behind one another to endless depths. To reproduce these millions of stars on charts proved to be utterly impossible.

The two observers then summoned the art of photography, recently so much improved, to their aid. Naturally they could not make use of the ordinary apparatus of the photographer; indeed, they were obliged to build a special telescope for their purpose. By means of clock-work, they succeeded in imparting to this a movement so prescribed and so regulated that the stars, though continuing in their unbroken course in the heavens, yet retain a stationary position with reference to the photographic plate. After many painstaking experiments, the enterprise was successful beyond expectation. Even the faintest of stars were plainly discernible on the plate, and in this manner more was accomplished in one hour than could be done by the old method of inscribing each star in many months.

These results incited to further progress. A new and very large telescope was constructed and directed toward the starry heavens. The plate now showed stars of the fifteenth magnitude, i. e., those whose light is so faint that only very few telescopes in all Europe can render them perceptible. In order to obtain this result, the plate, notwithstanding its extreme sensitiveness, had to be exposed to the light of these stars for fully an hour. If one were to carefully examine such a plate, or rather a cliché made therefrom, doubts might perhaps arise as to whether some of the little points thereon might not have been occasioned by particles accidentally present on the original plate. Such doubts might well be entertained, but Messrs. Henry have succeeded in meeting them in a most ingenious manner. After having exposed the plate for an hour, they shifted its position a trace to the right, and